The Global System for Mobile Communications (GSM) General Packet Radio Service (GPRS) and Enhanced Data for Global Evolution (EDGE) is intended to enable a service subscriber to send and receive data in an end-to-end packet transfer mode without utilization of network resources in the circuit-switched mode. GPRS, EDGE and 3rd Generation (3G) packet radio services permit the efficient use of radio and network resources when data transmission characteristics are i) packet based, ii) intermittent and non-periodic, iii) possibly frequent, with small transfers of data, e.g. less than 500 octets, or iv) possibly infrequent, with large transfers of data, e.g. more than several hundred kilobytes. User applications may include Internet browsers, electronic mail and so on.
FIG. 1 is a schematic diagram of a typical cellular communication system for use in explaining the operation of the present invention. As illustrated in FIG. 1, a cellular communication system 100 includes a number of cells 102–116, each defining a radio coverage area established by a fixed site base station located within each cell. For example, as illustrated in FIG. 1, cell 102 defines the radio coverage area established by a base station 118 located in cell 102, and similarly, each of the remaining cells 104–116 define an associated radio coverage area established by a corresponding base station (not shown) located within each of cells 104–116.
As a mobile station, such as a cellular telephone device, for example, travels with a user from position x to position y in cellular communication system 100, the mobile station continuously monitors the signal characteristics from the base stations of cells 102–116 and, based on certain selection criteria, selects a cell from which to receive and transmit packet data with a network 120 through the associated base station. For example, while the mobile station is positioned in cell 114, if the signal characteristics from cell 114 are such that, based on the selection criteria, cell 114 is selected as the “best” coverage area, cell 114 is considered to be the “serving cell”, or cell from which the mobile station transmits and receives packet data.
The mobile station continues to monitor the signal characteristics from cells 102–116, and, as illustrated in FIG. 1, as the mobile station subsequently moves along the marked path from position x to position y, the mobile station moves from the coverage area associated with cell 114 into the coverage area associated with other cells, such as cells 116 and 106 for example. Once the signal characteristics from another cell, cell 116 for example, are such that cell 116 is considered the best cell, the mobile station reselects cell 116 as the serving cell, until the signal characteristics from another cell, cell 106 for example, are such that cell 106 is considered the best cell, and the mobile station reselects cell 106 as the serving cell, and so on.
Since a user of a mobile station may be traversing the radio coverage area associated with more than one of cells 102–116, a known ordinal integrity mechanism, specified in the current GSM specification, GSM 04.60, “Digital cellular telecommunications system (Phase 2+); General Packet Radio Service (GPRS); Mobile Station (MS)—Base Station System (BSS) interface; Radio Link Control/Medium Access Control (RLC/MAC) protocol”, (European Telecommunications Standards Institute (ETSI), European Standard (Telecommunications Series), is incorporated into packet data services to ensure the ordinal integrity of data flow when a mobile station leaves the coverage area of one cell and enters a new cell.
FIG. 2 is a schematic diagram of a partial representation of a data plane for GPRS/EDGE. As illustrated in FIG. 2, both a mobile station 200 and a network 202 include equivalent hierarchically related control layers, such as a logical link control (LLC) layer 204, a radio link control (RLC) layer 206, a medium access control layer 208 and a physical layer 210. Packet data that is transmitted between mobile station 200 and network 202 is organized at logical link control layer 204 for transmission within logical link control frames, with each logical link control frame varying in size up to 1530 octets. As one logical link control frame logically propagates downward through the data plane, it is divided into multiple radio link control data blocks, with each radio link control data block being 22 to 54 octets. Each radio link control data block is in turn interleaved over four physical layer bursts with added redundancy.
As illustrated in FIGS. 1 and 2, if mobile station 200 is located at position x and is sending GPRS/EDGE data to network 202 via the serving cell, i.e., cell 114, cell 114 receives and acknowledges all of the radio link control data blocks that comprise the logical link control frames corresponding to the GPRS/EDGE data transmitted to network 202. If, while serving cell 114 is receiving a logical link control frame from mobile station 200, mobile station 200 reselects to a new serving cell, serving cell 116 for example, mobile station 200 reselects to cell 116, aborting the current temporary block flow on cell 114 and re-establishing the temporary block flow on cell 116, which now becomes the serving cell.
According to the known ordinal integrity mechanism, once the temporary block flow is re-established on new serving cell 116, mobile station 200 reorganizes its radio link control data block transmission window and begins by sending the first radio link control data block in the last unacknowledged logical link control frame. As a result, all of the radio link control blocks corresponding to the last logical link control frame being transmitted while cell 114 was the serving cell would have to be re-transmitted, despite the fact that some of those radio link control blocks may have been correctly received in serving cell 114. For example, if 53 radio link control blocks were needed to transmit a single logical link control frame, and radio link control blocks 1–50 were successfully transmitted up to the point at which reselection is performed, radio link control blocks 1–50 would be discarded and would therefore have to be retransmitted to the new selected cell to continue transmission of the logical link control frame.
In this way, in an environment in which rapid cell reselection is likely to occur, such as a congested urban environment for example, the known ordinal integrity mechanism produces a severe reduction in data throughput because of the periodic discarding of properly received information upon each reselection to a new serving cell.
Accordingly, what is needed is an improved method and apparatus for reducing the impact of cell reselection on user data transfer rates.